I. Document scanners utilizing linear Charge Coupled Devices (CCDs) are known. In a single color linear document scanner, the document, such as film, is scanned one line at a time onto the single array of CCD elements. Trilinear CCD comprises three CCD arrays (for example, one for red, one for green, and one for blue wavelengths) in order to achieve color imaging. These document scanners employ a fixed position single-pass imaging lens to image the document onto the CCD array, such that the object and image planes lie on opposite sides of the imaging lens if folding mirrors are not used. By single-pass imaging lens we mean that the light rays can pass through the imaging lens only once. This renders the imaging system of the document scanner quite long (.congruent.100 mm, for example). Such document scanner is shown schematically in FIG. 1A.
In addition, in order to properly scan a document, such as film, on a photosensitive medium such as a CCD, the imaging system of the document scanner has a very high resolution (typically, about 50 to 200 cycles/mm). The high resolution requirement corresponds to a small depth of focus of the imaging lens. Therefore, in order to establish a stable optical performance, it is necessary to maintain the object plane, the image plane, and the imaging lens in precise and rigid mechanical relationship with one another. This assures a well-focused image that is of the proper magnification, and without image distortion caused by rotation of either the document or the CCD during the scanning process. This mechanical rigidity is achieved by a housing that holds the imaging lens, the document (film, for example) and the CCD in proper spatial relationship with one another. This mechanical rigidity is difficult to achieve at reasonable cost because the tolerances on stability of the document, CCD and the imaging lens must be very tight (.+-.10 .mu.m). Many manufacturers build their systems out of many separate parts. As a result, the combination of tolerances on the housing parts require tight control of manufacturing processes, making the housing expensive to produce.
Furthermore, the housing is usually mounted to a base that provides necessary mounting for other components of the document scanner. As described above, the imaging system (from the document support to the CCD array) of such document scanners is relatively long. The length of the imaging system causes it to be susceptible to misalignment due to mechanical stresses induced by the base mechanical structure, thermal excursions and humidity variations. In addition, when the mounting components for the imaging lens and the CCD array are molded out of plastic, mechanical distortions (warping) of the mounting components are introduced during cooling of the molded components. These distortions can cause misalignment of the imaging lens, CCD, and document with respect to one another, causing image degradation.
In addition, the assembly of the imaging system is time consuming and expensive for the following reasons.
The fabrication of imaging lenses results in some variation of focal length among the imaging lenses. This results in the image being out of focus. The imaging lenses may also introduce erroneous tilt of the image plane relative to the object plane. These changes must be compensated during the assembly of the imaging system through adjustment of the object to the image plane distance (by shifting the CCD towards or away from the object plane), and tilt of the CCD. The CCD array(s) must be rotated to a position such that the line of CCD elements is perpendicular to the motion of the document in order to avoid skewing of the resultant image. The location of the center of the CCD array and the center of the scanned line of the document must be located on or close to the optical axis of the imaging lens. Thus, it is necessary to be able to move the imaging lens along the optical axis, and adjust the position of the CCD array in as many as 5 different ways (three directions--x, y, z and two tilt angles .theta.x, .theta.y). Further, the change in the focal length of the imaging lens results in a magnification change. This magnification change needs to be corrected or minimized by moving the imaging lens along its optical axis with respect to the object plane. When this magnification change is compensated for by this axial displacement of the imaging lens, the image is no longer focused on the CCD. Thus the CCD needs to be moved again to provide a sharp image on the CCD. This in turn creates a change in magnification. The process of moving the imaging lens and moving the CCD is repeated over and over again until both the required magnification and a relatively sharp image on the CCD array are obtained. Thus, the assembly of the imaging system requires simultaneous adjustment for best focus and magnification, and the adjustment process is iterative. Since both imaging lens and CCD must be moved, the time required for the iterations is an important consideration in a high volume alignment environment. The iterative process and the number of adjustments required to assemble a document scanner of this type is lengthy and expensive.